1. Field of the Invention
The present invention relates to electro-optical scanners and more particularly to scanning systems and related enabling mechanisms suited to detection, acquisition and/or tracking of target objects within a generally circular or conical field-of-view (FOV), based on sensing radiation energy, typically in the infrared (IR), visible light, or ultraviolet (UV) region, emitted by a source integral to the scanner and reflected from the objects. The invention is suited to such purposes as: forward-looking infrared (FLIR) active sensing; and detection, acquisition or tracking of terrestrial, airborne and space-based objects.
2. Description of the Related Art
A conventional technique of radiation energy pattern scanning for both passive scanners, i.e., where radiation is self-emitted by detected objects, and active scanners, i.e., where impinging radiation is reflected by detected objects, employs a raster-scan format analogous to the usual scanning of a television screen. The line/frame raster scan format is widely used for its compatibility with two-dimensional detector arrays such as charged-coupled device (CCD) type. However, there are a number of inherent drawbacks and disadvantages. Elaborate gating techniques are required, for example, to handle line and frame rate retrace. Simple image rotation presents complex and time-consuming processing tasks to the operating hardware and software. A small target, whose total height approximates the height of a scanned line, can be sampled only once during each field or frame. Furthermore, with raster scan the resolution is inherently constant over the entire scanned region, although in many applications the requirement for resolution increases strongly toward the center of the region where target acquisition activity tends to concentrate. For this reason, one raster scan approach of known art provides for "super-scan", an auxiliary system for effectively increasing scanning density by a factor of several times over a small region in the center of the field-of-view, for enhancing target acquisition and tracking resolution. Such a two-step (or multi-step) technique imposes burdens of complexity in hardware and supporting software.
Polar scanning offers an attractive alternative from several viewpoints including increased center resolution, continuous scanning progression, and inherent ease of data processing, e.g. data interpretation relating to simple rotation. In addition, an alternate method of defining and modeling a field-of-view is provided, facilitating use of alternate methods and mathematical models during image analysis.
Polar scanning has been proposed and utilized in particular specialized types of field applications, usually in some type of radial mode, as exemplified in well known radar and tracking techniques utilizing a rotating directional antenna. U.S. Pat. No. 3,916,196 to Thomson exemplifies radial type scanning: a 360 degree annular zone is scanned by a first reflecting plane, inclined in front of a fixed optical lens and rotating at twice the speed of a second mirror assembly having a pair of reflecting planes at right angles to each other in an optical path leading to a fixed linear array of IR detector elements.
A nearly hemispheric field of view is provided according to U.S. Pat. No. 4,703,179 to Motooka, wherein a multi-detector, multi-sided focal plane array, rotating on a platform, provides for varying the shape of arrays from a circle sector to rectangular.
A form of sector scanning is also shown in U.S. Pat. No. 4,188,531 to Pusch.
A form of circular scanning is disclosed in U.S. Pat. No. 3,554,628 to Kennedy.
U.S. Pat. No. 5,151,812 to Watson, entitled "Passive Energy Diametric Scanner," which is incorporated herein by reference, discloses a method and device for diametric scanning, wherein scanning of a circular FOV is performed in a continuous incremental manner along successive diametric paths each starting at a point at the circumference and proceeding through the FOV center to the diametrically opposite circumferential point. Target objects in the FOV are detected by means of their self-emitted (i.e., passive) radiation.
There are many applications where, because of high background, low signal-to-noise ratio, or operational constraints, it is essential or desirable to actively search for and track detected objects by scanning a total FOV with a beam of infrared, visible light, or ultraviolet radiation energy having a cross-section several orders of magnitude smaller than the FOV, so that the beam successively irradiates instantaneous fields-of-view (IFOV) which are much smaller than the total FOV. Object(s) within an irradiated IFOV will reflect a small portion of the radiation which may, in principle, be detected either by so-called monostatic detection means collocated with the beam source or by so-called bistatic detection means located elsewhere. If the beam source emits radiation pulses, range and range rate may be measured as with conventional microwave radar sets. In such applications, particularly where target objects tend to cluster towards the center of a total FOV or where a priori tracking information from a relatively coarse sensor is used to cue a finer-tracking sensor, diametric active scanning offers advantages over raster active scanning by providing a smoothly increasing scanning density from the periphery of a field-of-view to its center. In raster-type active scanning systems, a small target whose total height approximates the height of a raster scan-line can be sampled only once during each field or frame. In contrast, diametric-type active scanning enables such an object, when centrally located, to be sampled once during each line scan. Such capability in an active target-tracking system can aid in updating aim-point errors more efficiently and expediently than is possible using raster-type scanning.
In raster-type active scanning, a "flying spot" sequentially scans, with constant dwell time, contiguous IFOV's. In contrast, in diametric-type active scanning, the dwell time along the path of each line-scan can be varied, The rate that the beam-spot moves can be slower towards the center and faster towards the periphery of the FOV.
Other advantages of using diametric scanning in active detection/acquisition/tracking systems are smooth, continuous and sequential IFOV irradiation, with an increasing scanning density of irradiation in the region of greatest interest, viz., towards the center of the FOV, and a decreasing scanning density towards the perimeter of the FOV, where generally there is less interest.
Data processing of a diametrically-imaged FOV can in some instances be more easily implemented than is possible using a raster-scan format, as for example in determining target object rotation.
With the exception of angular offset of one line to the next, all diametrically-imaged scan lines are identical. For example, all lines are bordered by adjacent lines throughout a FOV. This characteristic may allow implementation of a multi-processing environment wherein each diametric line is independently processed by a separate dedicated processor, each processor communicating with neighboring processors so as to collectively perform a high-speed image analysis of the FOV.